The use of plasmas for etching substrates such as semiconductor silicon chips or wafers at controlled rates and with controlled etched profiles is well known in the art. At its simplest level, plasma etching tools basically provide for the mounting of a photoresist masked wafer between two parallel plates within an evacuated chamber. A gas such as CF.sub.4 is introduced to the chamber and the plates are oppositely driven by a radio frequency power source at a power level of 10 to 10,000 watts, creating an electronic discharge therebetween. This ionizes the gas into a highly corrosive plasma which etches the wafer through the interstices of the mask, to produce the etched design and profile desired.
This process of plasma etching semiconductor chips has several disadvantages. First, it is necessary to utilize a significant amount of energy to convert the confined gas into a plasma. The radio frequency voltage also produces a voltage bias at the wafer surface. This bias can be advantageous, but it is generally difficult to control or regulate, thus resulting in inefficient use of the plasma for etching and a lack of desirable control in directing the plasma during the etching process. The high power radio frequency supplies are also relatively expensive, compared to comparable DC power supplies.
A further significant problem in the foregoing process is the non-uniformity of density of the plasma near the wafer. These types of processes generally rely upon high discharge energies and the constant throughput of gas to maintain an appropriately dense plasma surrounding the substrate.
Attempts have been made to overcome these problems. For example, one known hot filament plasma etching technique utilizes a volume magnetic field to control the plasma within an etching chamber. This system generally includes an evacuated chamber having a filament electrode and a spaced anode plate located within the chamber. The filament is heated by electric current from a power supply so as to emit or "boil off" electrons, thereby creating an electrical discharge between the filament and the opposed anode. The anode itself may be separately biased to control the potential of the plasma relative to ground. Partial control of the plasma thus created is accomplished by use of a volume magnetic field produced by electrical current flowing through magnet coils located above and below the etching chamber.
While this device does control the plasma in the chamber to a certain extent, the use of such electromagnets produces a volume magnetic field where magnetic field lines of varying strength extend throughout the entire plasma volume. Accordingly, there is no overall uniformity of plasma density. Instead, plasma is concentrated consistent with the field lines. Less uniformity and undesirable variations in the etching profiles can result. Moreover, since a volume magnetic field is produced, both ions and primary or ionizing electrons will constantly migrate out of the region surrounding the wafer. This reduces the efficiency of the system by increasing the amount of gas necessary to maintain a given plasma density and by increasing the amount of energy expended to maintain the plasma through generation of ionizing electrons to replace those escaping. Significant amounts of energy are also required to generate the electromagnetic field.
A major problem with any plasma etching system wherein plasma and ionizing electrons are lost by migration, as discussed above, is that a substantial amount of ionizing electrons and plasma must be continually produced. In particular, plasma ion loss requires more gas to be used with resulting higher gas pressures. This has the effect of not only increasing the migration of ions out of the device through collisions, but also the shortening of the mean free path of the ionizing electrons. A shorter mean free path increases the random motion of corrosive reactant ions, and produces more isotropic etching. This reduces the ability to etch the narrow profile features of the semiconductor desired.
Accordingly, it is one object of this invention to provide improved methods and apparatus for plasma etching a substrate, such as a semiconductor wafer.
Another object of the invention is to provide improved methods and apparatus for plasma etching of semiconductor chips or other substrate surfaces at controlled rates and with highly defined and controlled etch profiles.
A further object of the invention has been to provide improved methods and apparatus for plasma etching semiconductor wafers without subjecting the wafer to induced electrical bias from the use of undesirably high powered radio frequency discharge power supplies.